The present document is based on Japanese Priority Document JP 2000-339458, filed in the Japanese Patent Office on Nov. 7, 2000, the entire contents of which being incorporated herein by reference.
1. Field of the Invention
The present invention relates to a vapor deposition method for forming an organic thin film used in organic electroluminescence (EL) devices. The present invention is also concerned with a vapor deposition apparatus for forming an organic thin film.
2. Description of the Related Art
EL devices are light emission devices utilizing an EL (electroluminescence) phenomenon such that a light emission occurs when an electric field is applied to a fluorescent substance, and they are generally classified into one in which a light emission occurs due to a collision of accelerated electrons with the radiative recombination center of the fluorescent substance placed in an electric field, and one in which electrons entering a semiconductor crystal having a fluorescence property cause a light emission at the pn junction interface. In the latter, a voltage is applied to a pn junction comprised of a semiconductor crystal, such as GaAs (red) or GaP (green), to combine a hole and an electron at the interface. This is a so-called light emission diode, and has a high light emission efficiency and therefore is now widely extended in a display application. In addition, in recent years, a blue light emission using GaN or AlGaN has been accomplished, and light emission devices are being spread in further applications including a full-color display. Thus, there exist a number of materials for EL devices, and, in addition to the above-mentioned semiconductors, many organic materials have been developed.
Organic EL devices basically have a structure such that organic thin films are stacked on one another, specifically a structure such that, on a glass substrate, an anode comprised of, for example, an indium-tin oxide (ITO), an organic hole transport layer, an organic light emission layer, and a cathode comprised of a metal are successively stacked on one another. For forming a thin film, the deposition for such organic EL materials has conventionally been conducted by a resistive heating deposition method (see, for example, Y. Hamada, C. Adachi, T. Tsutsui, S. Saito, J. Appl. Phys. Jpn., 31, 1812 (1992); and Y. Ohmori; A. Fujii, M. Uchida, C. Morishima, K. Yoshino, Appl. Phys. Lett., 62, 3250 (1993)). This deposition method is inherently a method employed when preparing metal thin films, for example, aluminum (Al) electrode wiring, and this is a method in which a substance to be deposited is placed in a boat made of a metal, such as molybdenum (Mo) or tantalum (Ta), and the boat is heated by passing a current through the boat to heat and evaporate the substance to be deposited, and the resultant vapor is deposited on the substrate to form a thin film. This method is easy, and excellent in film thickness controllability, and, especially when this method is applied to deposition in vacuum, a thin film having a clean surface can be prepared.
However, the resistive heating deposition method which has been employed for the deposition of metals is not necessarily suitable for the deposition of organic materials. Specifically, the resistance heating is conducted by passing a current through a filament provided in a bottom portion of a boat made of a metal, and, for this reason, a material contained in the boat is locally heated from the bottom side of the boat. Therefore, the whole of the material cannot be uniformly heated. Further, when a plurality of deposition sources are used, it is extremely difficult to keep constant the heating state of materials contained in the respective deposition sources. In addition, the temperature of the boat being heated rapidly rises to a temperature as high as about 1,000xc2x0 C., and hence the temperature control for the boat is difficult. When an organic material having a vaporization temperature of about 100 to 400xc2x0 C. is heated in such a boat, the organic material is partially crystallized, decomposed, or deformed in the boat and the remaining organic material is evaporated all together, thus causing the control of the deposition rate to be difficult.
By the vapor deposition method for forming an organic thin film of the present invention, an organic material is heated by the infrared light and is vaporized to be deposited on a substrate. Therefore, the vapor amount of the organic material, that is, the deposition rate is controlled by adjusting the heating temperature for the organic material, so that the deposition can be conducted at an arbitrary deposition rate in the range of temperatures as low as 100 to 400xc2x0 C. Thus, a deposition rate can be easily controlled with excellent response.
In addition, in the vapor deposition method for forming an organic thin film of the present invention, the crucible is provided with a projection inside the crucible on the side from which the infrared radiation enters. Therefore, a part of the infrared light undergoes total reflection off the projection and is then radiated to the inside of the crucible, so that the vapor of the organic material is prevented from being solidified again on the inner sidewall and inner upper surface of the crucible and the organic material is uniformly heated, thus making it possible to obtain a stable deposition rate.
Further, by the vapor deposition apparatus for forming an organic thin film of the present invention, the organic material can be uniformly heated, making it possible to stably control the deposition rate.
There is a need for a vapor deposition method for forming an organic thin film, which is advantageous in that the deposition rate can be easily controlled and deposition can be conducted with stable high controllability. Accordingly, the present invention has been made with a view toward solving the above-mentioned problem.
The vapor deposition method for forming an organic thin film of the present invention comprises heating an organic material by irradiation with an infrared light (infrared radiation), and allowing the organic material vaporized by the heating to be deposited on a substrate.
In the vapor deposition method for forming an organic thin film of the present invention, the infrared light directly heats the organic. Therefore, the heating temperature for the organic material is arbitrarily set at about 100 to 400xc2x0 C., making it possible to achieve uniform heating for the organic material.
Further, the vapor deposition apparatus for forming an organic thin film of the present invention comprises a closed container, an infrared light emitter for emitting an infrared light, a crucible for containing therein an organic material, wherein the crucible is comprised of a material capable of transmitting the infrared light, and a holder for supporting a substrate on which the organic material is deposited.